1. Field of the Invention
The invention relates generally to safety systems that limit the amount of radiant energy that can be emitted from an open optical fiber or a transmitter port in an optical communication link. More particularly, the invention relates to a safety system that can be incorporated on laser based optical fiber link cards, where the safety system is operative to detect open link failures (for example, an optical fiber in the link which has been disconnected or broken), to reduce the laser's radiant energy output (or shut it off) when an open link failure is detected, to periodically check to determine if the failure is corrected, and to restore full continuous power to the laser upon determining that the link is operationally safe.
2. Description of the Related Art
Many types of laser based devices and systems, having a wide range of applications, such as in medical technology, in communications and computing technology, etc., are becoming increasingly well known and commercially available.
The lasers used in many of these devices and systems are often capable of producing powerful outputs that are potentially harmful to both people and equipment. As a result, many types of safety devices for use in conjunction with laser based equipment, and standards designed to ensure that laser based equipment may be safely operated, have been developed and continue to evolve.
For example, U.S. Pat. No. 4,423,726, to Imagawa et al, describes a safety device for a laser ray guide (used in the performance of laser surgery) that employs the combination of a lense and a laser ray receiving element, to detect a failure of the laser ray guide. Reflected laser light is used to operate the Imagawa, et al safety system. Although suitable for detecting failures in the local laser based system in which it is used, Imagawa et al does not teach a safety system that controls the laser; does not teach a safety system that shuts the laser down (or limits its power output) upon detecting a failure; and does not teach a safety system suitable for use in performing safety control over long distances, such as over fiber optic links used in communication and/or computing systems.
Safety systems employing reflection to detect fiber failures are both impractical and far too complex (i.e., large and costly) to be used with optical data communication systems because of the difficulty of being able to distinguish the reflection due to a link failure at any point along the link from the reflections due to connectors, splices and the receiver/detector at the end of the link.
Another example of a prior art safety device for a laser based system is taught in U.S. Pat. No. 4,543,477, to Doi et al. This safety system is used to detect problems in an optical transmission fiber of a medical laser application. The system utilizes reflected laser light to control a shutter mechanism which blocks the light when a problem is detected.
Like Imagawa et al, Doi et al does not teach a safety system that controls the laser; does not teach a system that shuts the laser down upon detecting a failure; and does not teach a system that is suitable for performing long distance safety control since reflection is again used as the means for detecting a problem.
In another patent issued to Doi et al, U.S. Pat. No. 4,716,288, a security device is described that detects failures in the transmitting fiber. The application is a high power medical laser used to perform surgery, and features means for detecting fiber damage (using reflection), which disables the laser (to prevent over heating the fiber) when a failure is detected. Although capable of disabling a laser, the Doi et al safety system taught in the 4,716,288 patent, like its predecessor in the 4,543,477 patent, still uses reflection to detect safety problems and therefore is not a system which is suitable for long distance applications.
Yet another example of a prior art safety system is the high power optical fiber failure detection system taught by Ortiz, Jr., in U.S. Pat. No. 4,812,641. The Ortiz, Jr. safety system is used in equipment that employs a high power pulsed laser to perform material processing. A break or leak in an optical fiber transmitting high power laser energy can be detected by the system, which then shuts down the laser beam delivery system when the optical fiber begins to fail. Separate sensing fibers and detectors are used by the system to detect breaks or leaks in the transmitting fiber.
The use of the separate sensing fibers and detectors called for by Ortiz, Jr., would be especially costly and problematic for long distance safety control applications. The need for the additional fiber links and sensing devices, and the necessity for the additional fibers to span long distances, etc., make such a system unsuitable for use in conjunction with many fiber optic links.
All of the above referenced patents deal with the transmission of power over very short lengths (less than a few meters) of optical fiber in which any failures in the fiber link would cause a substantial change in the reflected power (typically an increase in reflectance) and would create both an exposure and a fire hazard. In contrast, a data communications link operates at much lower power levels and over much longer distances (for example, two kilometers would not be unusual) and a failure in the optical link (for example, a disconnected mechanical splice) would create only a viewing hazard and very little change in the amount of reflected power. The impracticality of using reflection combined with the vastly different environments of the current application versus the applications discussed in the referenced patents would make a totally new type of safety control system desirable.
In addition to being able to detect failures in laser based systems and effectively turn a laser off when a fault is detected; it is also desirable in many applications to be able to power the laser back up and resume operations after the condition causing the fault has been corrected.
No safety control systems are known that combine a link failure detection capability, that easily and cost effectively detects link failures over the distances spanned by a particular link, with a control system that is capable of reducing the laser's radiant energy output to a safe level (or shutting laser off) when a failure is detected. Additionally, no safety control systems are known that are also capable of periodically checking to determine if a detected failure is corrected, and causing full continuous power to the laser to be restored upon determining that the link is operationally safe.
With the increasing use of fiber optic technology to provide solutions to performance and packaging problems associated with present day computer interconnect applications, communication applications, etc., small laser based feature cards, such as the optical fiber link card described in copending patent application Ser. No. 07/462,681, filed on even date herewith and assigned to the same assignee as the present invention, are being developed. Copending patent application Ser. No. 07/462,681, is hereby incorporated by reference.
From a safety and product certification point of view, it would be desirable if a safety system could be provided that would make each individual card, such as the card described in the above referenced copending patent application, "fail safe", i.e. certifiably safe at other than a total system level.
Many countries require certification of the "product" with respect to laser light emissions. Prior art laser based optical link subassemblies have a dependency on the "box" they are in to maintain compliance. If a safety system could be devised that could be self contained on each card, then the card would become the "product" that needs to be certified; not all the different models of the boxes that it is used in.
The laser safety standards and certifications referred to hereinabove can be very stringent for an open fiber in an optical communication link. For example, the European IEC laser safety standards for class 1 operation limit the amount of power that can be emitted from an open fiber to a maximum level of approximately -8 dBm, which is far below the design point for optimal performance of the link to which the card in the referenced copending patent application is coupled.
In view of the present and evolving standards applicable to laser based optical fiber link systems, including optical fiber link cards, etc., it would be desirable to be able to reduce the certification requirements for such systems as a whole by providing the aforementioned "fail safe" capability at the feature card level. Any safety control system that could provided such a feature would have to be compact enough to fit on an individual card, would need to be compatible with other components on the card (in terms of power requirements, noise, etc.), and would need to be easily and cost effectively operable independent of the length of the fiber link to which the laser on board the card is coupled.
Accordingly, it would be desirable if a safety control system could be provided that (1) operates, in a self contained fashion, as part of an optical fiber link card designed to be coupled to an optical fiber link; (2) operates in cooperation with an identical safety system on the other card included in a bidirectional optical fiber link; (3) provides sufficient safety features to allow the card to conform to all existing worldwide safety regulations for class 1 operation, and to remain class 1 under a single fault condition (class 1 is defined herein as in the International Electrotechnical Commission (IEC) Standard publication number 825, published in 1984); (4) easily and cost effectively detects link failures (such as a disconnected mechanical splice), over the distances spanned by a particular link; (5) reduces (or shuts off) the laser's radiant energy output, when a link failure is detected; (6) periodically checks to determine if the failure is corrected; and (7) restores full continuous power to the laser upon determining that the link is operationally safe.